The present invention relates to lawn mowers and in particular to battery powered lawn mowers.
Commercially available types of battery powered lawn mowers usually comprise a cutting deck mounted on four wheels. A battery is mounted on the cutting deck together with an electric motor. The motor rotatingly drives a cutting blade which is located below the cutting deck. A handle is attached to the rear of the deck and which is used by an operator to maneuver the lawn mower. An electric switch is mounted on the handle near to the section gripped by an operator so that the operator can squeeze a lever which operates the switch. A cable runs from the battery to the switch, a second cable runs from the switch to the motor and a third cable runs from the motor to the battery to form an electric circuit. Activation of the switch completes the circuit and thus energizes the motor.
One problem with battery powered mowers is that run time between recharges of the mower is limited and dependent on the size of the battery.
As the size of the battery increases its weight and hence the weight of the overall mower increases, thus reducing the ease of handling and maneuverability of the mower. Therefore, the size of the battery is a compromise between the length of the run time between recharges and the maneuverability of the mower. Therefore, the amount of power which can be stored in the battery and which is available to drive the motor is limited.
It is desirable to maximize the running time of the mower between recharges of the battery.
One way of achieving this is by minimizing the amount of energy wasted through the electric circuit between the battery and motor when the motor is running. The amount of energy lost through electric cables is dependent on the length of the cables. Therefore, it is desirable to minimize the length of the cables of the circuit in order to reduce the amount of energy lost through the cables. This can be achieved by mounting the switch on the deck of the mower, thus avoiding the use of the cables which run from the deck along the length of the handle to the switch.
U.S. Pat. No. 3,564,186 discloses an electric mower having a switch mounted on the deck. The switch is operated by a bowden cable which connects at the other end to a lever mounted on the handle. In battery powered mowers the output voltage of the battery is low, often less than 20 V. Therefore, in order for the motor to generate sufficient power when it is running, it must draw a considerable current from the battery through the interconnecting cables. The amount of power lost through the interconnecting cables is dependent on the square of the value of current multiplied by the resistance of the interconnecting cable I2R. Therefore, a small variation in the resistance of the interconnecting cable can have a large effect on the amount of energy lost through the interconnecting cables in a battery powered mower. The problem with the design of the mower disclosed in U.S. Pat. No. 3,564,186 is that the switch is connected to the motor via wires which are require to be attached to the terminals of the switch. This introduces resistive barriers into the electric circuit and thus wastes energy. The amount of energy wasted would be considerable if the design of the mower described in U.S. Pat. No. 3,564,186 was powered by a battery.
Furthermore, due to resistance in the electric circuit there would be a voltage drop between the battery and the motor, thus reducing the voltage supply to the motor and hence the amount of power the motor is able to produce. The requirement to attach wires to the switch also increases production time and costs.
It is desirable to use regenerative breaking in an electrical lawn mower when switching the motor off. Regenerative braking can be induced in the motor when it is rotating after the electrical power to the motor has been disconnected by electrically connecting the terminals of the motor together. Regenerative braking uses the energy stored within the motor to slow the rotation of the motor. This is commonly implemented in a battery powered mower by adding a third cable which runs from the motor to a switch mounted on the handle to enable the switch to connect together the terminals of the motor when it disconnects the electricity supply to the motor. This adds extra cost in providing the additional wiring.
The energy used for the regenerative braking of the motor is lost through the regenerative braking circuit. The amounts of energy can be considerable which result in large potentials and currents being generated. This results in arcing which can damage the components of the regenerative braking circuit. It is therefore desirable to dissipate the energy generated in the regenerative circuit as efficiently as possible. Therefore, in contrast to the electric circuit which is formed whilst the motor is running, it is desirable to introduce into the regenerative electric circuit a resistance through which the energy can be dissipated. Common designs of regenerative circuits and the methods by which a resistance is incorporated are complex and thus expensive.
The object of the present invention is to provide a battery powered mower which overcomes the above problems.
According to the present invention there is provided a battery powered lawn mower which comprises a cutting deck;
an electric motor mounted on the deck;
a battery mounted on the deck and which is in electrical connection with the motor;
a switch mounted on the deck and which forms part of the electrical connection between the battery and the motor, the switch comprising terminals, with the terminals which are in electrical connection with the motor and through which electrical current passes when the motor is running extend from the switch and connect directly to the terminals on the motor.
By extending the terminals of the switch from the switch directly to the terminals of the motor the terminals of the switch act as busbars, providing a solid conductor with a large cross section between the switch and the motor which is capable of transmitting large currents efficiently and with minimal loss between the switch and the motor. Furthermore, the number of interconnections, such as solder joints, connectors etc., in the electrical circuit between the switch and motor is reduced thereby reducing the amount of energy lost through the electrical connection. This is a particularly important feature in a battery powered electric mower in which a large current is drawn through the interconnecting conductors. Any reduction of the resistance of the interconnecting conductors can make a significant reduction in energy losses and thus improve the run time of the mower. Furthermore, because the resistance of the interconnecting conductor is reduced, the voltage drop across the conductors is also reduced. Therefore, the voltage across the electric motor is increased enabling the motor to provide a higher power output due to the increased voltage supply across its terminals. It also simplifies the production process by reducing the number of electrical connections which need to be manufactured and components parts which need to be used, thus saving time and money. It also produces a more robust connection between the switch and the motor. This is an important feature for a mower as the motor and switch and their interconnections are subjected to a considerable amount of vibration when the mower is used.
Preferably, the switch comprises internal connectors which extend from within the switch to form the terminals of the switch. By having one continuous piece of conductive material forming the internal connectors, the terminals of the switch and the interconnection between the switch and the terminals of the motor, the number of interconnections in the electrical circuit between the switch and the terminals of the motor are further minimized. Furthermore, the internal connector, the terminal of the switch and the interconnection between the switch and the terminal of the motor are formed as a single component.
The terminals of the switch can be attached to the terminals of the motor using a nut and bolt, clips, electrical connectors or clamps. However, it is preferable that the terminals of the switch are permanently connected to the terminals of the motor. By permanently attaching the terminals of the switch to the terminals of the motor by soldering for example, ensures that a good electrical connection is made between the two and which continues to provide a good connection over a period of time.
To remove the interconnection between the terminals of the switch and the terminal of the motor to further reduce the number of resistive barriers, the motor could preferably comprise electrical brushes wherein the terminals are integrally formed with the brushes. The brushes can be held in plastic holders. This provides a cheap and simple mechanism for supporting the brushes. To further simplify the construction, the holders can be integrally formed with the body of the motor.
Preferably the switch is located adjacent the terminals of the motor. This reduces the distance between the switch and the terminals, thus reducing the length of the conductor between the switch and the terminals of the motor, thus reducing costs and the amount of energy lost through the interconnection.
In order to reduce the energy losses through the electrical connection between the battery and the motor yet further, the terminals of the switch which are in electrical connection with the battery may be extended from the switch to connect directly to the terminals of the battery.
The terminals of the switch can be used to secure the switch in position. This provides a simple and easy way of securing the switch onto the mower. It further reduces the cost of the mower as separate mounting means for the switch are not required.
Electrically powered lawn mowers often comprise an over current protection device to provide protection to the motor and the battery. Ideally, the terminals of the over current protection device are connected directly to the terminals of the switch. This helps to minimize the number of connections required within the circuit. Over current protection devices include fuses, thermal cut out devices and safety key switches. The over current protection device can be integrally formed with the switch. Alternatively, the switch can comprise a socket for receiving the over current protection device. Often the over current protection device is located remotely from the switch, motor and battery. This requires additional wiring between the over current protection device and the other components within the electric circuit. By locating the over current protection device next to the switch, the length of the interconnecting conductors is reduced, minimizing the amount of energy lost through the interconnecting connectors.
To further simplify the construction of the switch, the socket can be integrally molded into the body of the switch, thus forming a compact design.
The over current protection device can be releasably attachable to the socket. This enables the operator to disable the lawn mower when it is not in use. The operator simply removes the over current protection device thus breaking the circuit and preventing the mower from being switched on. When the operator wishes to use the mower, he re-inserts the over current protection device to complete the electrical circuit.
The body of the switch may be integrally formed within the motor housing. This results in a simplified and robust construction further simplifying the manufacturing process of the mower.
The design of the switch can comprise a relay. However, a mechanical switch is more desirable as they are cheaper to produce and require no electrical power in order to operate. One such design of switch comprises a moving connector mounted on a sliding element which is capable of sliding over the fixed connectors and which is capable of making electric connection between differing adjacent internal connectors depending on the relative position between the sliding element and the internal connectors.
Ideally, the switch is configured to switch between two modes of operation, a first mode wherein the switch completes the electric circuit between the battery and the motor and a second mode wherein the electric circuit is broken and the two terminals of the motor are connected together to cause regenerative breaking in the motor when the motor is rotating. One such design of switch comprises a first terminal which connects to both a terminal on the motor and to a terminal on the battery, a second terminal which connects to the second terminal on the motor and a third terminal which connects to the second terminal of the battery, wherein the first mode of operation, the switch, electrically connects the second terminal to the third terminal and in the second mode of operation the switch electrically connects the first terminal of the switch to the second terminal. By constructing the switch in this manner, it produces a simple and compact structure which enables the switch to be easily changed between its two modes of operation. The third terminal connects to the second terminal of the battery via a fourth terminal which is connected to the third terminal via the current over load device.
The regenerative breaking circuit formed when the switch is in the second mode of operation may comprise a resistance. Such resistance can be created by the insertion of a resistor or a coil of resistive wire.
A second design of switch which incorporates a resistance within the regenerative breaking circuit may comprise a first terminal which connects to both a terminal on the motor and a terminal on the battery, a second terminal which connects to the first terminal via resistance, a third terminal which connects to the second terminal of the motor and a fourth terminal which connects to the second terminal of the battery wherein in the first mode of operation the switch electrically connects the third terminal to the fourth terminal and in the second mode of operation the switch electrically connects the second terminal to the third terminal of the switch. The fourth terminal may connect to the second terminal of the battery via a fifth terminal, which is connected to the fourth terminal via a current over load device to provide protection.
Ideally, the switch comprises a connector in electrical contact with at least one terminal. This provides a simple power outlet to which can be attached other electrical devices mounted on the lawn mower. The power outlet connects directly to main electric circuit of the mower and can be configured such that it is switched on only when the motor of the mower is activated. The design of the switch can be simplified by constructing the body of the connector integrally with the body of the switch.
The switch can be operated by a mechanical link such as a rod or, preferably a bowden cable.